1. Field of the Invention
The invention relates to the field of production of olefins from alcohols. More particularly, it relates to the field of producing olefins by way of dehydrating alcohols in reactors under either adiabatic or isothermal conditions.
2. Background of the Art
Ethene is an important raw material in the world today and is known to be useful as a raw or starting material to prepare products as wide-ranging as packaging, detergents, chemical weapons agents, and anesthetics. It is also used as a welding gas and fruit ripening agent. It is particularly important in the field of plastics production, and a majority of the global ethene production goes toward this purpose. Because of its carbon-carbon double bond it is highly reactive, enabling a wide variety of useful polymerization products.
Methods to prepare ethene have been known since the late 18th century, when it was discovered that it could be made by passing ethyl alcohol over a heated catalyst. This method was common for decades until it was discovered in the 1950's that it could more economically be obtained via steam cracking of naphtha, which converted longer-chain hydrocarbons to shorter-chain hydrocarbons and introduced unsaturation. This is a laborious and very energy-intensive process, which was propagated primarily during a time period when petroleum reserves seemed endless and relatively inexpensive. At the present time, however, petroleum-based processes may be considered to be less desirable and, therefore, new and improved methods to produce ethene are being sought.
One method that has been described is the preparation of ethene by dehydration of ethyl alcohol, wherein ethyl alcohol vapor is passed over solid catalysts maintained at high temperature in multitubular, isothermal reactors. The isothermal conditions are maintained by circulating a heating fluid externally to the tubes, thereby indirectly heating the ethyl alcohol vapor. A problem with this method, however, is that such heating fluids necessarily exhibit a high boiling point and high thermal stability, and include certain organic liquids and low melting inorganic salts. Few organic liquids can be maintained at a temperature greater than 370° C. without degradation, while molten salts, which may be heated to 550° C., are likely to be solid at temperatures below 150° C. This change of state may result in highly problematic obstruction when plant equipment either fails or is shut down. Molten salts may cause equipment corrosion, while equipment materials that can withstand temperatures greater than 450° C. include only certain expensive steels. Furthermore, if molten salts and organic vapors come into direct contact, flammability and safety issues may arise. Finally, the multitubular reactors, which offer the increased heat exchange area needed for the highly exothermal dehydration reaction, represent significant initial capital outlays, yet enable only a limited throughput rate.
Another method is disclosed in U.S. Pat. No. 4,134,926 (1979). That method includes the catalytic dehydration of ethyl alcohol to ethene in a fluidized catalyst bed. The use of the fluidized bed, at reaction temperatures of at least 700° F. (370° C.), preferably at least 750° F. to 900° F. (that is, 400° C. to 482° C.), and a reaction time from 1 to 10 seconds, is described as increasing overall yields to greater than 99 percent.
Yet another approach to catalytically preparing ethene from ethyl alcohol is disclosed in U.S. Pat. No. 4,232,179 (1980). In that process an ethyl alcohol feed is introduced to the catalyst simultaneously with a “sensible” heat carrying fluid, which may be selected from, for example, a part of the effluent from the reactor used as a recycle stream; steam supplied by an external source; other adequate fluids for the process; or any combination thereof. Adiabatic reactors containing a fixed catalyst bed are used, singly, in parallel or in series, enabling catalyst exchange, maintenance, without detriment to process continuity. Because of the heat carrying fluid, multitubular reactors are not necessary and the heating fluid requires no independent circulation. Higher temperatures may therefore be used, which facilitate higher space velocities. The patent alleges reduction in by-product and coke deposition on the catalyst, but the relatively high amount of water reduces energy efficiency by requiring water separation and cleanup, which in turn requires additional processing equipment.
U.S. Pat. No. 4,396,789 (1983) discloses another process relating to dehydration of a low molecular weight alcohol to form ethene in fixed adiabatic reactors. This process uses a combination of a co-feed of ethyl alcohol and steam, at a temperature of 400° C. to 520° C. and a pressure from 20 to 40 atmospheres (that is, 2.03 to 4.05 megapascals (MPa)), in a plurality of adiabatic reactors containing a fixed bed catalyst, to accomplish the dehydration of the ethyl alcohol. Subsequent washing and purification steps are described as producing a higher purity ethene.
Despite efforts to discern an effective method of producing an olefin via dehydration of an alcohol, desirably under adiabatic conditions which become more economically favored as process scale increases, it has generally been found that large co-feeds of water or other heat-carrying diluents increase yields, but also undesirably increase costs, particularly those relating to energy and capital. At the same time, reducing water content requires more adiabatic reactors and increases byproduct production and fouling. Those skilled in the art therefore continue to search for adabatic processes whereby alcohols may be dehydrated to form olefins as conveniently and inexpensively as possible, and whereby problems arising from production of byproducts and coke deposition are minimized. The present invention provides such a process.